Measuring the impedance in a large system in which conductive components of various kinds contribute to the resistive as well as reactive components can present a substantial practical problem. A prime example of such a system is an electrical transit system which is energized by direct current. It is important to know the resistance of the system in, for example, a circuit breaker recloser. If a circuit breaker has opened as the result of an overload, it is desirable to close the breaker again as soon as possible but only if the condition of the circuit meets certain requirements. These requirements can be defined in various ways but the fundamental condition which must be met is the absence of a severe fault in the system which would cause an extreme current surge as soon as the breaker is reclosed with resulting damage or immediate reopening of the breaker. The existence of a fault of this type can indicate a dangerous condition along the line.
Devices for evaluating the condition of a system in the context of automatic reclosers are shown in the following patents:
U.S. Pat. No, 4,454,555, DeLacy PA1 U.S. Pat. No. 4,232,232, DeLacy et al PA1 U.S. Pat. No. 4,164,773, DeLacy
These patents involve rather complex and quite expensive devices. Furthermore, the measurements and evaluations which are made are not necessarily indicative of actual conditions.
As further background, it should be noted that the systems under consideration involve considerable lengths of wire or rail, or both, by which DC power is supplied to electric cars. The system can be a trolley system in which power is supplied on an overhead cable with one or more rails being used as the return conductor, or it can be a third rail system in which DC power is supplied on an insulated rail and one or both of the running rails are used as the returns. A fourth rail is also used in some systems as the return.
In any of these, there can easily be two miles or so of conductor between the measuring location and the next substation. Also, there may be a car along the section of interest with its motors connected to the line as well as lights and other accessory equipment. This circuit thus represents a resistive value and also displays considerable inductance, commonly on the order of 15 to 20 millihenrys. If the circuit is in good condition, i.e. if there are no "crowbar" shorts along the line, the breaker can be reclosed and this dead load can be picked up without difficulty, the deadload commonly being in the order of 100-500 amps. It is, however, essential to rule out the possibility of a serious fault before reclosing which requires rapid and reliable measurement of the DC resistance of the circuit without any effect from the circuit inductance.
A problem of somewhat similar magnitude but in a different context arises in connection with the measurement of the DC resistance of a very large distribution transformer winding. The normal way of making such a measurement is to apply a voltage to the transformer with some form of ohmmeter attached and wait until the inductive effect has subsided, the measured impedance at that time being, in theory, the DC resistance. One problem with that technique is that it takes a very long time to make the measurement because the inductance values are so large. A further problem is that the meter reading approaches the DC value asymptotically and it is quite difficult to determine when the correct reading has been reached. Similar problems arise in other large and distributed-inductance systems.